The impact of the background bubble layer on reverberation-derived scattering strengths in the low to moderate frequency range

Abstract

The impact that a near-surface, range-independent background or persistent bubble layer may have on the derivation of sea surface backscattering strengths from reverberation measurements is examined. A simple ray model is proposed to account for the refractive and attenuating effects of the bubble layer and is used to calculate the modified insonification of the air–sea interface. This simple approach is validated against a highly accurate numerical solution. Scattering at the interface is handled via first order small perturbation theory. The combined propagation/scattering model is exercised in the low- to moderate-frequency range in order to examine bubble-induced modifications to sea surface backscatter calculations. Results indicate that the refractive effects due to the background bubble layer significantly enhance scattering levels as a function of wind speed. Furthermore, reasonable variations in the background bubble spectrum are shown to yield scattering levels that compare quite well with explosive source data from the 7th Critical Sea Test (CST-7) and the Chapman–Harris experiment. The model also predicts a weaker grazing angle and frequency dependence than evidenced in either data set.